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Bureau of Mines Information Circular/1986 



Coal Mine Roof Instability: 
Categories and Causes 

By Noel N. Moebs and Raymond M. Stateham 




UNITED STATES DEPARTMENT OF THE INTERIOR 



i 



Information Circular 9076 

// 



Coal Mine Roof Instability: 
Categories and Causes 

By Noel N. Moebs and Raymond M. Stateham 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Model, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 







Library of Congress Cataloging in Publication Data: 



Moebs, Noel N 

('oal mine roof instability. 

(Bureau of Mines information circular ; 9076) 

Bibliography: p. 12. 

Supt. of Docs, no.: 1 28.27: 9076. 

1. Coal mines and mining— Safety measures. 2. Mine roof control. 
3. Coal— Geology . I. Statcham, Raymond M. II, Title, III. Series: 
Information circular (United States. Bureau of Mines) ; 9076. 



^PN^^SrtJt" 622s [622'. 334] 86-600014 



4 



i 



CONTENTS 



Page 



Abstract 1 

Introduction 2 

Previous work 3 

Categories of roof failures 3 

Type S — stress related 4 

Subtype S i , in situ stress 4 

Subtype S2, induced stress 6 

Type G — geologic defects 6 

Subtype G\, low rock strength 6 

Subtype G2 > moisture sensitivity 7 

Subtype G3 , bedding plane spacing 8 

Subtype G4 , minor structures 9 

Subtype G3, major structures 11 

Discussion 11 

References 12 

Appendix. — Glossary 13 

ILLUSTRATIONS 

1. Subtype Si roof failure associated with topographic "notch" 4 

2. Subtype Si roof failure attributed to high lateral regional stress 5 

3. Subtype S2 roof failure attributed to mining-induced stress 6 

4. Subtype Gi roof failure attributed to low rock strength 6 

5. Subtype G2 roof failure attributed to moisture— sensitive roof rock 7 

6. Subtype G3 roof failure attributed to thinly laminated strata 8 

7. Subtype G4 roof failure attributed to sllckensides 9 

8. Subtype G4 roof failure attributed to kettlebottoms 9 

9. Subtype G4 roof failure attributed to clay vein (clay dike) 9 

10. Subtype G4 roof failure attributed to paleochannel (roof roll) 10 

11. Subtype G4 roof failure attributed to joints 10 

12. Subtype G4 roof failure attributed to pinchouts 10 

13. Subtype G4 roof failure attributed to concretions 10 

14. Subtype G4 roof failure attributed to faults 10 

TABLE 

1 . Tabulation of roof-fall occurrences 4 





UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


o 


degree MN/m^ meganewton per square meter 


in 


inch psi pound (force) per square inch 



COAL MINE ROOF INSTABILITY: CATEGORIES AND CAUSES 

By Noel N. Moebs and Raymond M. Stateham 



ABSTRACT 

Coal mine roof failure Is categorized according to character, trend, 
or pattern of occurrence. Two principal categories of failure are pro- 
posed — geology related and stress related. Geology-related failure In- 
cludes both llthology and structure. Each of several subcategories re- 
flects the probable cause of failure and thereby provides a basis for 
the selection of appropriate techniques for reducing the Incidence of 
failure. These control techniques, depending on local conditions, may 
Include supplementary support, destresslng, reduction of mine air humid- 
ity, or a change la the customary support methods. 



^Geologist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 
^Geophysicist, Denver Research Center, Bureau of Mines, Denver, CO. 



INTRODUCTION 



Roof-bolting practices are largely em- 
pirical, having evolved from much experi- 
mentation and trial and error. Bolting 
theory still is more descriptive than 
mathematical. The subsurface environment 
surrounding a coal mine is complex, and 
the failure of bolted mine roof is diffi- 
cult to describe fully. Nonetheless, the 
qualitative identification of the con- 
ditions leading to roof failure can pro- 
vide some basis for modification of mine 
design that should reduce instances of 
failure under prevailing conditions. 
Most operators are aware of the sometimes 
subtle and sometimes pronounced changes 
in roof conditions that occur during a 
mining operation, and the difficulty of 
either predicting these changes or re- 
sponding with the appropriate support or 
mine design to prevent roof failure. 
However, the recognition of a general 
characteristic of individual falls or a 
pattern of multiple falls usually will 
offer some clue as to the cause and, 
therefore, the appropriate remedial 
action. 

The purpose of this paper is to provide 
the mine operator with a guide for deter- 
mining the probable cause of persistent 
roof failure based on the occurrence, 
character, and distribution of the indi- 
vidual falls in a mine, and to offer sug- 
gestions as to how additional roof fail- 
ure might be avoided. The diagnosis of 
roof failure is based chiefly on the 
careful examination of a mine roof-fall 
map, and recognition of certain charac- 
teristic patterns of distribution or 
modes of failure. This requires diligent 
documentation of a significant number of 
falls, as occasional widely distributed 
falls seldom are sufficient for analysis 
and in most mines would not indicate a 
need for action. Occasional failure can 
result from faulty materials or improper 
bolt installation, but these factors are 
outside the scope of this paper. Roof- 
fall documentation at the mine should in- 
clude the following: 

1, A mine map showing all roof falls 
and indicating not only the location, but 
also the approximate size and dimensions 
of the falls. This map will then provide 



information for determining trends of 
roof failure. 

2. A mine map with surface topography 
superimposed for correlating roof fall 
occurrences to topography features. 

3. A map showing positions of super- 
jacent or subjacent mining. 

4. A map showing roof structures such 
as clastic dikes or sandbodies, as well 
as major geologic features (faults, dense 
jointing, etc.). The combined informa- 
tion from maps of this type can be ex- 
tremely useful in the diagnosis and pre- 
vention of roof failure. 

The significance of roof-fall patterns 
became discernible upon an examination 
of numerous roof falls in scores of coal 
mines , followed by a review of supplemen- 
tary geologic information on each prop- 
erty and an interview with underground 
personnel. Some patterns of roof falls 
were erratic or difficult to explain, and 
some were found to correlate with a 
transition in bolt type, mining cycle, or 
entry design. This paper summarizes the 
significance of roof-fall patterns at- 
tributable to structural or stress condi- 
tions and offers suggestions for remedial 
action. 

Before describing roof-fall patterns 
and their significance, it must be empha- 
sized that any supplementary information, 
such as maps showing the character and 
thickness of roof strata, or discrete 
roof structures such as rolls or clay 
veins , may help explain the occurrence of 
falls and aid in determining the appro- 
priate remedial action. 

The improper installation of bolts con- 
tributes to roof failure but, with re- 
spect to stress and geologic factors, is 
a lesser influence on roof-fall occur- 
rence. There are several reasons for 
this condition. First, improperly in- 
stalled, mechanically anchored bolts can 
be detected prior to failure by torque 
measurements; poorly installed full- 
column, resin-grouted bolts are not so 
easily detected. However, even these im- 
properly installed bolts tend to be rela- 
tively effective support devices because 
some portion of the bolt's length is usu- 
ally well bonded to the rock. As little 



as 12 to 18 in of bond can provide an an- 
chor that will hold a load greater than 
the yield strength of the steel bolt. On 
the basis of personal observations and 
experience, the authors believe improper 
installation is a lesser problem when 
compared to the stress and geologic in- 
fluences discussed in this paper. This 
is especially true in coal mines because 
of the following: 

1 , Coal mines are located in relative- 
ly weak, bedded sediments where adverse 
stress and geologic conditions severely 
affect ground stability. 



2. Coal mines generally use rotary 
drilling equipment that provides consist- 
ent hole sizes and spin rates so that the 
grout is mixed adequately for proper bolt 
installation. 

3. Mechanical bolts are tested ac- 
cording to procedures defined by safety 
regulations . 

Aside from inappropriate support, im- 
proper installation, configuration or 
size, or mining sequence for existing 
conditions , most roof failures can be at- 
tributed either to high stresses or to 
geologic defects in roof structures. 



PREVIOUS WORK 



Several schemes for classifying mine 
roof have been attempted, chiefly for the 
purpose of predicting the occurrence of 
roof falls from a knowledge of local geo- 
logic features. For example. Weir (V)^ 
described six kinds of roof falls as fol- 
lows: shale dusting or slaking, sand- 
stone rolls, concretions, slabbing, clay 
seams, and massive. These categories of 
falls probably predominate in the spe- 
cific area of Indiana studied but are of 
limited usefulness for the entire region. 
Hylbert (2^) proposed a classification of 
roof falls based on the structural and 
compositional character or roof at a mine 
in eastern Kentucky. This scheme proved 
useful in projecting trends of roof con- 
dition as an aid in anticipating problem 
areas in advance of mining. Some infer- 
ences can be drawn from this scheme re- 
garding the appropriate support method 
for each of the four types of roof condi- 
tions described by Hylbert. 

Patrick and Aughenbaugh O) have de- 
vised a classification based only on the 
geometry of a roof fall. The categories 
are dome, arch, minor, and sloughing. 
This simplified scheme is intended to 
expedite the reporting of roof falls 



independent of local conditions and to 
serve as a first step toward developing a 
means of predicting the occurrence and 
extent of future falls. While some in- 
ferences as to the cause of failure can 
be drawn from a geometric classification 
only, further usefulness in analyzing 
roof control problems is very limited. 

The classification of roof proposed by 
the Bureau of Mines requires somewhat 
more information than that needed in the 
works of other investigators (3); how- 
ever, it should provide for broader us- 
age, supply a sound basis for diagnosing 
the underlying causes of roof failure, 
and indicate some appropriate means of 
reducing the rate of failure. 

Often, a simple inventory of individual 
roof-fall occurrences will indicate the 
probable cause of failure, as shown in 
table 1, which is based on examples from 
four selected mines in western Pennsyl- 
vania and northern West Virginia. This 
method, however, requires extensive map- 
ping of roof falls and is too broad in 
classifying, although it may be useful 
in conjunction with the classification 
scheme proposed here. 



CATEGORIES OF ROOF FAILURES 



For simplicity and clarity, the illus- 
trations used here to designate various 

■^Underlined numbers in parentheses re- 
fer to items in the list of references 
preceding the appendix at the end of this 
report. 



types of roof-fall patterns are sche- 
matic. Often the cause of roof failure 
is obscure and cannot be determined with 
any degree of certainty, or only through 
prolonged and sophisticated research. 
The following categories are intended 
only to provide mine operators and/or 



TABLE 1. - Tabulation of roof-fall occurrences 





At inter- 


Between 


At minor 


>1 




Principal cause of failure 


Mine 


sections 


inter- 
sections 


structures 


pillar 
length 


Total 


and support required 


1: 

Number 


163 


23 


4 


10 


200 


Low-strength roof rock, 


Pet. .. 


82 


11 


2 


5 


100 


requiring bolted headers, 


2: 

Number 












straps, and trusses. 


4 


10 





115 


129 


High lateral stresses (cut- 


Pet. .. 


3 


8 





89 


100 


ter roof) , requiring posts 


3: 

Number 












and crossbars. 


21 


12 





350 


383 


High lateral stresses (cut- 


Pet... 


5 


3 





92 


100 


ter roof) , requiring 
posts, crossbars, and 


4: 

Number 












steel sets. 


10 





34 





44 


Minor structures, chiefly 


Pet.. . 


23 





77 





100 


clay veins, requiring 
straps or header block. 



mine safety personnel with the means to 
make a preliminary and rapid assessment 
of roof problems using observable pat- 
terns of roof failure and geologic infot- 
mation. While underground options always 
are limited, some early remedial measures 
might be attempted once the probable 
cause of failure has been established. 
Some of these measures are suggested. 
All roof-fall patterns have been divided 
into either of two categories — Type S or 
stress related, and Type G or geology 
related — and a schematic illustration of 
each subtype is provided for quick refer- 
ence. Type G includes both lithology and 
structure-related failure to facilitate 
classification. Caution must be exer- 
cised in using only the illustrations 
since the accompanying description of re- 
lated surface or subsurface features may 
be equally diagnostic. 

TYPE S— STRESS RELATED 

Subtype S^, In Situ Stress 

In the northern Appalachian coal re- 
gion, one of the most common and easily 
recognized types of roof failures occurs 
beneath narrow stream valleys in areas 
of high relief (fig. 1). It is referred 



to by various miners' terms such as 
"pressure falls," "snap top," and "cutter 
roof." These falls can be recognized by 
comparing their occurrence with a map 
showing surface stream valleys where top- 
ographic relief is at least 100 ft. It 
has been estimated by several operators 
that when mining beneath or near such 
stream valleys, severe roof-fall prob- 
lems will develop 90 pet of the time. 



^WWWXi!W>.VAV-«"'-ll.i.lWW- 




Topographic "notch" or 
~,, valley at surface" 
.„,„,(Not to scale) . 







Scole, ft 



FIGURE 1. - Subtype $1 roof failure associated with 
topographic "notch." 



Examination of these falls shows little 
if any evidence that jointing contributed 
to the failure, and even the most compe- 
tent roof rock is subject to this type 
of failure. The falls frequently result 
from a concentration of high lateral com- 
pressive stresses, a phenomenon described 
by several authors (4-^) . They are rec- 
ognized underground sometimes by an audi- 
ble snapping sound immediately after min- 
ing, or by the development of a steeply 
dipping shear or "cutter" at the inter- 
section of rib and roof within a few 
hours to a week or two after mining. 
This type of failure usually develops be- 
tween intersections but may progress 
along cutters across one or more inter- 
sections for perhaps several hundred 
feet. 

In some instances of S^ roof failure, 
further falls have been prevented by the 
installation of supplementary angle bolts 
to intersect the cutter plane and anchor 
above the pillar (fig. 1). Currently, 
angle bolting is being tested in at least 
three mines for this purpose. The imme- 
diate installation of support to prevent 
any yielding of roof is commonly recom- 
mended. More severe S] failure will re- 
quire posts and crossbars or possibly 
roof trusses. It may be virtually uncon- 
trollable with crushing of posts or cribs 
leading to massive high roof falls. 

Roof failure that clearly is caused 
by high lateral compressive stress, but 
that is not limited to occurrences be- 
neath valleys and occurs somewhat random- 
ly, is included here (fig. 2). It is 
characterized by the development of a 
cutter along the rib line within a few 
weeks of mining, roof cracks, and a typi- 
cally sudden roof fall if not well sup- 
ported. Kripakov (9^) describes cutter 
roof failure in detail, discusses current 
control methods , and suggests some new 
alternatives. Competent shale roof fails 
under these pressures as readily as does 
softer laminated roof rock. The pattern 
of these falls commonly shows a preferred 
north-south trend in several of the 
U.S. coal regions. Blevins (4^) reported 
a similar north-south failure condition 
in the Illinois coal basin. The falls 



.Absence of pronounced topographic -'//-, 
"notch "or valley at surface 




Z - — Underclay-^_j^^^~~~~^^;^-^~__~:j-_^ 

5 3 

Scale, ft 

FIGURE 2. - Subtype S 1 roof failure attributed to 
high lateral regional stress. 

generally begin between intersections and 
may zigzag around pillars to follow a 
north-south trend, or may show no clear 
directional preference. Subtype S^ falls 
can be distinguished from those attrib- 
uted to pillar punching by the absence 
of accompanying floor heave and pillar 
spalling, and by the evidence of a direc- 
tional trend. 

S] roof failure includes roof falls 
that tend to occur chiefly at the bound- 
aries of multiple-entry mains. These 
falls typically begin as a cutter along 
the rib line and commonly have been at- 
tributed to "abutment pressures." The 
exact cause, however, remains disputable. 
When adequate pillar support is provided 
and there is no evidence of roof deflec- 
tion in the central zone, the pressure 
arch theory does not seem to offer an 
adequate explanation of failure, and in 
situ stresses are suspected. A change in 
entry orientation, where possible, has 
proved to be more beneficial than a modi- 
fication of pillar design, but experience 
in controlling this problem is very lim- 
ited. Destresslng of the rock surround- 
ing an entry by means of roof or rib 
slotting or induced caving of an adjacent 
entry has been attempted with varying de- 
grees of success. Destresslng, however, 
while sound in theory, is risky, and 
requires equipment not always available 



in a mine. Normally, supplementary 
support as described in S^ failure is 
suggested. 

Subtype S2> Induced Stress 

Induced-stress roof failure (S2 — fig- 
ure 3) nearly always can be related to 
superjacent or subjacent second mining or 
to a squeeze or pressure override from a 
panel that has not fully caved. The map 
overlay technique, such as that described 
by Ellenberger (10) , will detect the ef- 
fects of multiseam mining on entry sta- 
bility, while pressure overrides nearly 
always occur within a few hundred feet 
of adjacent pillar extraction or pillar 
stumping. Geologic and mining correla- 
tions are useful in resolving the problem 
of induced stresses, especially with re- 
spect to the massive character of strata 
that transfer overburden weight onto pil- 
lars or abutments. Induced stresses from 
multiseam mining or pressure overrides 
lead to various combinations of floor 
heave, pillar spelling or deformation, 
pillar punching into floor and roof, and 
cutter roof, with the actual failure of 
roof sometimes occurring late in the se- 
quence of events. 

The remedy for S2 roof failure due to 
pressure overrides probably lies with 
improved pillar extraction and caving 
or with oversized pillars. Ground con- 
trol problems resulting from multiseam 
mining may be alleviated by following 
extraction sequencing guidelines as 
described by Britton (11). An increase 



in conventional bolting, strapping, or 
posting seldom is of much value in pre- 
venting further failure, therefore steel 
sets generally are needed. 

TYPE G — GEOLOGIC DEFECTS 

Categories of roof failure attributed 
to geologic defects or geologic character 
of rock are divided into five subtypes, 
as follows: 

G] - Low rock strength. 

G2 - Water sensitivity. 

G3 - Bedding-plane spacing, 

G4 - Minor structures, 

G5 - Major structures. 

Subtype G^, Low Rock Strength 

This category of roof includes all roof 
rock that is relatively soft, usually 
poorly laminated, and low in RQD (Rock 
Quality Designation), point load, and 
compressive strength. The physical prop- 
erties for subtype G^ (fig. 4) commonly 
would fall below the following values: 

Point load index 0.3 MN/m^ 

Shore hardness 20 

Compressive strength.... 2,500 psi 




3 

I I 

Scole, ft 



Floor heave 



FIGURE 3. - Subtype $2 roof failure attributed to 
mining-induced stress. 



Limestone 




Scale, ft 



FIGURE 4. - Subtype G^ roof failure attributed to 
low rock strength. 



Low-Strength rocks include most clay- 
stone (especially the "drawslate" that 
overlies the coalbed) and underclay or 
seat earth. These rocks generally are 
not self-supporting for normal entry 
widths, tend to fall from the roof soon 
after the supporting coal has been re- 
moved, and therefore must be supported 
as quickly as possible after exposure. 
Bolted headers or straps usually are 
needed, and trusses are useful in severe 
cases, where the deadweight of collapsing 
roof is not excessive; otherwise, posts 
and crossbars become necessary. Full- 
column resin-grouted and tensioned resin- 
anchored bolts have been used success- 
fully at some localities to support this 
type of roof, but further assessment is 
needed. With most mechanical bolts, 
there is a problem of tension bleedoff in 
soft rock due to anchor slippage; in 
addition, segments of soft rock tend to 
fall from between bolts, and the re- 
inforced beam effect of the roof is 
disrupted. 

Systematic drilling, core logging, and 
point-load testing of drill core are 
useful in delineating areas where low- 
strength roof rock can be anticipated. 
This type of rock commonly occurs between 
coal splits where the roof of the lower 
bed consists of the underclay of the up- 
per bed; it is widespread over the Pitts- 
burgh coalbed in the Upper Ohio River 
Valley. 

Subtype G2 , Moisture Sensitivity 

Moisture sensitivity is used here to 
indicate a significant reduction in the 
strength of roof rock from exposure to 
high humidity or water, such as is com- 
monly found in the mine environment. 
(See figure 5.) The effects of moisture 
sensitivity consist of a progressive 
softening or slaking, whereby rock grad- 
ually disintegrates and eventually re- 
verts back to a mudlike unconsolidated 
condition. The slaking process is one 
of moisture absorption, expansion, and 
softening. In a humid mine atmosphere, 
slaking may be a gradual process, the 
effect of which often is measured in 
months or years, resulting in a slow 
failure of roof by attrition as the 



Limestone 



—^ Roof bolts— "^ 




Clay shale or 


^ 


clays' 


One 






" Y ^ '- T - 




~ 


n 


— 


^ _ 


— 


— 


--> — V, 


_ -_ 


_ 


^ X 


_/ ^-x^ 


--^ ^ 


^y-i 


i \ 


__ 


^^^^^ I 





Entry 




-Underclayr. 



Scale, ft 

FIGURE 5. - Subtype Q>2 roof failure attributed to 
moisture-sensitive roof rock. 

dislodging of small fragments leads to 
larger falls of roof. The integrity of 
an entire mass of roof can be destroyed 
by this process. In mechanically sup- 
ported areas, roof-bolt tension bleedoff 
occurs as the rock immediately against 
the bolt plate becomes softened, or when 
the anchor slips. Moisture-induced roof 
failure generally is most pronounced and 
severe near shaft bottoms and along in- 
take air courses. The rate of roof slak- 
ing is greatest during the humid summer 
months when roof rock commonly is wet 
with condensation. 

Where moisture-sensitive roof rock is 
thinly interbedded with moisture-stable 
rock, the bond between the two types of 
strata is weakened, leading to strata 
separation. Sandy roof rock rarely is 
affected by moisture except where the 
sand grains are poorly cemented and the 
rock reverts to a loose sand. 

The principal type of moisture-sensi- 
tive roof rock in the Appalachian coal 
region consists of a poorly laminated 
lumpy claystone containing numerous 
slickensides , which sometimes is known as 
clod in miners' terminology. Generally a 
simple water- immersion test or exposure 
outdoors will determine the relative 
moisture sensitivity of roof rock samples 
and the extent to which this may become a 
problem underground. The prevention or 
control of moisture-induced roof failure 
can be accomplished by any of the follow- 
ing four principal methods: 



1. Head coal . — The uppermost 4 to 6 in 
of coalbed, if left unmined, serves as a 
moisture barrier and may prevent slaking 
of shale roof. 

2. Sealing . — Sealing entails the coat- 
ing of exposed roof with an impervious 
layer of material to exclude moisture. 
The sealant can consist of an asphalt- 
or latex-base material with little physi- 
cal strength, or it may consist of a 
cement-base gunite with fiber additive 
sprayed over a bolted wire mesh for added 
strength and reinforcement. The effec- 
tiveness of these measures depends large- 
ly on the quality of the sealant and the 
care with which it is applied. Sealing 
of large areas of roof invariably is a 
costly procedure. 

3. Artificial support . — Some form of 
supplementary support almost always is 
required to reduce the failure of mois- 
ture-sensitive roof. The options are nu- 
merous and need not be discussed here. 
Limited experience with full-column resin 
bolts indicates their superior ability to 
hold soft, slickensided, claystone-type 
roof, as opposed to mechanical bolts, 
which lose tension as moisture attacks 
the rock at the bolt head and anchor. 
As disintegration due to moisture pro- 
gresses, a larger area of roof than that 
immediately above the bolt plates re- 
quires support, and this usually is pro- 
vided by bolted headers, straps, or mesh. 
Long-term disintegration usually necessi- 
tates trusses or posts and crossbars to 
support an increasinly large amount of 
deadweight from sloughing roof. 

4. Air tempering . — The term "temper- 
ing" as used here refers to a modifying, 
adjusting, or stabilizing of mine air 
moisture and temperature, usually with a 
resulting decrease in humidity levels and 
humidity fluctuations. Air tempering has 
been accomplished through the use of 
water sprays, heaters, and cooling units, 
depending on the season, but at high 
cost and with limited success. A passive 
and more cost-effective method of temper- 
ing mine air is through the use of air- 
tempering rooms or entries. Here, fresh 
air is passed through a set of rooms or 
multiple entries where it is cooled and 
loses moisture in the humid season and, 



to a lesser degree, is warmed and absorbs 
moisture in the winter months. This eli- 
minates large fluctuations in humidity 
and temperature before the air enters 
haulageways and other active sections of 
a mine. Provision must be made for some 
roof deterioration in the tempering rooms 
or entries, which should be included in 
the original mine design. This method of 
air tempering was assessed recently in 
both field and laboratory investigations 
( 12 ) . It was concluded that the use of 
air tempering entries may be cost effec- 
tive in controlling roof disintegration, 
depending on conditions at a particular 
mine. 

Subtype G5 , Bedding-Plane Spacing 

A bedding plane in laminated roof 
strata constitutes a potential plane of 
separation (G3 — figure 6). The weaker 
the bonding along the bedding plane, the 
more likely a roof separation will occur 
as the coal underneath is removed. Weak 
bonding usually results from an abundance 
of mica flakes, clay, or coal material 
along the bedding plane; the more closely 
spaced the bedding planes or thin lamina- 
tions, the more difficult it will become 
to form a beam of the immediate roof un- 
less it is strongly reinforced with roof 
bolts. Thinly laminated roof strata of 
both low strength and closely spaced bed- 
ding planes, such as a "rash" of coal, 
claystone, and shale, are certain to be 
troublesome roof to support, Stackrock, 



Massive sandstone * ' 



Lominated:' 
bondsfone:: 



-* 't <, * «. « 


Wi 


1 


m 




[p:^ 




rUnderclay: 



Scale, ft 



FIGURE 6. - Subtype G3 roof failure attributed to 
thinly laminated strata. 



a miner's term for very thinly laminated 
sandstone, does not respond well to 
conventional bolting and is prone to fall 
on exposure. Roof falls attributed to 
closely spaced and poorly bonded lamina- 
tions usually occur at intersections, 
where the greatest span of roof is ex- 
posed, but sometimes occur randomly 
wherever the roof support or installation 
is inadequate or defective. Falls of 
immediate roof due to a high density of 
poorly bonded bedding planes tend to de- 
velop first as roof sag because the roof 
bolts are not anchored into overlying 
competent thick-bedded strata. As the 
strata in the immediate roof separate 
along bedding planes and sag, a slip- 
page along the planes also occurs. This 
alone, however, is unlikely to prevent 
eventual roof failure unless some support 
is provided by longer bolts anchored in 
overlying competent strata. Severe sag 
of thinly laminated strata that does 
not respond to fully grouted, tensioned 
resin-anchored, or longer bolt calls for 
the use of roof trusses, posts and cross- 
bars, or entry narrowing where feasible. 

The sagging of laminated strata often 
results in a tension fracture along the 
center of the entry roof caused by the 
bending moment. As sagging progresses. 



I I 



fracturing of the 
eventually destroys 
leads to a general 
collapse. 



roof occurs , which 
roof integrity and 
disintegration and 



'I I 

Limestone T 







J 



FIGURE 7. - Subtype G4 roof failure attributed to 
s lickensides. 




Scale, ft 



FIGURE 8. - Subtype G4 roof failure attributed to 
kettlebottoms. 



Subtype G4 , Minor Structures 

Falls of roof attributed to minor geo- 
logic structures generally are recognized 
by a minor structure that is exposed in 
the roof or fall or lies adjacent to the 
fall. Minor structures include virtually 
any geologic feature other than a normal 
parallel layering of roof strata. These 
include slickensides (fig. 7), kettlebot- 
toms (fig. 8), clay dikes (fig. 9), pa- 
leochannels (fig. 10), joints (fig. 11), 
pinchouts (fig. 12) , concretions (fig. 
13), and faults (fig. 14). Most minor 
structures constitute a discontinuity in 
the normal beamlike structure of mine 
roof and thereby have a weakening effect. 



J _± 



H 



T 



Limestone 







Scale, ft 



FIGURE 9. - Subtype G4 roof failure attributed to 
clay vein (clay dike). 



10 



Sandstone 




FIGURE 10. - Subtype G4 roof failure attributed to 
paleochannel (roof roll). 




3 
1 I 

Scale, ft 



^ _ Underclay- r ~:_~ T ^^~ ~ :C ~ - ^ Z ZJr 



FIGURE 13. - Subtype G4 roof failure attributed to 
concretions. 



Joints 




Shale - 




Scale, ft 

FIGURE 11. - Subtype G4 roof failure attributed to 
joints. 



■■•■ Sandstone . 



. =::7v"^T^."vT^^^7Tr7rv," 


!^Roof.'--';.''\ 
■bolts ■'" 


'^ig;— - 


■,/.■;.■';■. 'Sandstone' .-,.' 


.fflW* 




,, 






J- 






p— /^ 





3 

I I 

Scole, ft 



-;j~_;^~ ~-Underclay-x_-^~-_~^~-^'XX-::r-c-c c ::: ~i 



FIGURE T2. - Subtype G4 roof failure attributed to 
pinchouts. 

The roof rock, around minor structures 
tends to fall soon after the supporting 
coal is removed and before a permanent 
support can be installed. 




Underclay 



Scale, ft 



FIGURE 14. - Subtype G4 roof failure attributed to 
faults. 

A multitude of minor structures have 
been encountered in Appalachian coal 
mines. Few have been fully described 
as to identify or effect on mine roof. 
Virtually all are either syngenetic or 
diagenetic in origin; that is, they are 
nontectonic, having been formed con- 
temporaneously with deposition or short- 
ly thereafter during compaction and 
consolidation. 

The actual character or identity of 
many minor structures can only be estab- 
lished through careful examination by an 
experienced geologist. The correlation 
between structure and roof falls, how- 
ever, can be fairly readily established 
by a systematic mapping of roof falls and 
minor structures, even though the iden- 
tity or trend of the structure is not al- 
ways apparent. 

Many minor structures such as paleo- 
channels , clay dikes, slickensides , 
slumps, rolls, and horsebacks, tend to- 
ward linearity, so that directional 



11 



trends of falls soon can be established 
and projected. Kettlebottoms and con- 
cretioas tend to occur sporadically and 
are particularly common in southern West 
Virginia. 

Intraformational joints are found in 
virtually every mine where thick massive 
strata occur. They commonly will form a 
boundary of a roof fall but do not con- 
stitute major causative factor. However, 
in eastern Kentucky, the so-called hill- 
seam, a weathered, valley stress-relief 
form of joint, has been the cause of nu- 
merous roof falls in drift mines. Joints 
are reported to play a much greater role 
in roof failure in the Western United 
States than in the Appalachian region. 

It would be impractical to attempt to 
describe all the minor structures and 
their variants. However, a knowledge of 
the nature of each structure above the 
exposed roof can be vital in preventing 
failure by tailoring the supplementary 
support to the local conditions. For ex- 
ample, neither kettlebottoms, concre- 
tions, nor jointing in mine roof are nec- 
essarily better supported by increasing 
the bolt length, while pinchouts may ben- 
efit from this procedure. The support of 
several minor structures, such as slick- 
ensides (slips), paleochannels , and clay 
dikes, seem to be improved when angle 
bolting is employed. Resin injection and 
dowelling have proved effective in many 
instances of consolidating clay dikes in 
the roof. Bolted straps and headers are 
widely used with virtually any type of a 



minor structure that constitutes a dis- 
continuity in roof strata. 

The severity of failure due to minor 
structures can be reduced when the gen- 
eral directional trend of these struc- 
tures can be established and entries can 
be turned to intersect them at a large 
angle as opposed to driving parallel to 
the structures. Every effort should be 
made to identify correctly the character 
and trend of troublesome structures on 
exposure, as they are not usually detect- 
able by exploratory drilling, occur er- 
ratically, and tend to fail without warn- 
ing when unexpectedly encountered during 
mine development. 

Subtype G3, Major Structures 



This category is intended to cover the 
large tectonic structures such as the 
faults and folds that occur along the 
eastern limits of the Appalachian coal 
region, the anthracite region of east- 
ern Pennsylvania, the Coosa and Warrior 
Basins of Alabama, the Illinois Basin, 
and some Western U.S. coal regions. 
Major tectonic structures, while recog- 
nized, are outside the scope of this 
paper and therefore are omitted from de- 
tailed discussion. However, major struc- 
tures in Illinois and their effect on 
mine roof have been described by Nelson 
(13) , and similar structures in Western 
U.S. coal regions were studied by Laird 
and Amundson (14). 



DISCUSSION 



The authors have presented a scheme for 
categorizing roof falls in mines based on 
causative factors and have indicated pos- 
sible means of upgrading roof support 
practices to prevent their occurrence. 
This scheme requires some data collection 
regarding the pattern and character of 
roof falls. Each setting has its own 
ground control problems , which can be de- 
scribed as stress effects and geologic 
defects. These two salient conditions 
can occur together, but they require a 
somewhat different approach in terms of 
improved roof support. Although improper 



extraction or support methods can con- 
tribute to roof failure, these factors 
usually can be identified by the absence 
of geologic defects or stress effects 
and the close examination of operating 
procedures . 

The proposed scheme for the diagnosis 
of roof falls and improvement of support 
clearly is only a framework in which the 
roof specialist of a mining company can 
begin to sort out his or her troubles. 
It is not always possible for a mine op- 
erator to allocate technical staff mem- 
bers full time for this kind of study. 



12 



But, if pursued conscientiously along 
with some sort of experimentation within 
the constraints of the approved roof 



support plan, it could contribute to 
accident prevention and reductions in 
cleanup and repair costs. 



REFERENCES 



1. Weir, C. E. Factors Affecting Coal 
Mine Roof Rocks in Sullivan County, Indi- 
ana. Proc. IN Acad. Sci. 1969, v. 79, 
1970, pp. 263-269. 

2. Hylbert, D. K. The Classification, 
Evaluation, and Projection of Coal Mine 
Roof Rocks in Advance of Mining. Min. 
Eng. (NY), V. 30, No. 12, 1978, pp. 1667- 
1676. 

3. Patrick, W. C, and N. B. Aughen- 
baugh. Classification of Roof Falls in 
Coal Mines. Min. Eng. (NY), v. 31, No. 
3, 1979, pp. 279-283. 

4. Blevins, C. T. Coping With High 
Lateral Stresses in an Underground Coal 
Mine. Pres. at Soc. Min. Eng. AIME Annu. 
Meeting, Dallas, TX, Feb. 14-18, 1982. 
Soc. Min. Eng. AIME preprint 82-156, 
1982, 7 pp. 

5. Roley, R, W. "Pressure-Cutting:" 
A Phenomenon of Coal-Mine Roof Failures. 
Mechanization, v. 12, Dec. 1948, pp. 69- 
73. 

6. Agapito, J. F. T., J. R. Aggson, 
S. J. Mitchell, M. P. Hardy, and W. N. 
Hoskins. Study of Ground Control Prob- 
lems in Coal Mines With High Horizontal 
Stresses. Paper in Proceedings of the 
21st Rock Mechanics Symposium (Univ. MO- 
Rolla, May 28-30, 1980). Univ. MO-Rolla, 
1980, pp. 820-828. 

7. Aggson, J. R. How To Plan Ground 
Control. Coal Min. and Process., v. 16, 
Dec. 1979, pp. 70-73. 



8. Lang, T. A. Theory and Practices 
of Rock Bolting. Trans. AIME, v. 220, 
1961, p. 335. 

9. Kripakov, N. P. Alternatives for 
Controlling Cutter Roof in Coal Mines. 
Paper in Proceedings of the Second Con- 
ferences on Ground Control in Mining, 
Morgantown, WV, (July 19-21, 1982). WV 
Univ., Morgantown, WV , 1982, pp. 142-151. 

10. Ellenberger, J. L. Hazard Predic- 
tion Model Development: The Multiple 
Overlay Technique. Pres. at Soc. Min. 
Eng. AIME Annu. Meeting, Chicago, IL, 
Feb. 22-26, 1981. Soc. Min. Eng. AIME 
preprint 81-16, 6 pp. 

11. Britton, S. G. Mining Multiple 
Seams. Coal Min. and Process., v. 17, 
No. 12, 1980, pp. 64-70. 

12. Cummings , R. A., M. M. Singh, and 
N. N. Moebs. Effect of Atmospheric Mois- 
ture on the Deterioration of Coal Mine 
Roof Shales. Min. Eng. (Littleton, CO), 
V. 35, No. 3, 1983, pp. 243-245. 

13. Nelson, W. J. Faults and Their 
Effect on Coal Mining in Illinois. IL 
State Geol. Surv. Circ. 523, 1981, 40 pp. 

14. Laird, R. B., and A. A. Amundson. 
Geologic Conditions Affecting Coal Mine 
Ground Control in the Western United 
States (contract J0145032, Goodson & As- 
sociates, Inc.). BuMines OFR 14-86, 
1985, 62 pp. 



13 



APPENDIX. — GLOSSARY 



1. Abutment pressure - In underground 
mining, the weight of rock above an exca- 



vation which has been 
adjoining walls. 



transferred to the 



2. Angle bolts - Bolts installed in the 
rock over an underground opening at an 
angle of less than 90° from vertical 
(usually 45°). They usually are in- 
stalled so as to penetrate a slip or 
shear plane and anchor over the adjacent 
rib (sidewall of the opening). 

3. Beam effect - The result of bolting 
of the mine roof whereby the bolted 
strata behave as a single beam, stabiliz- 
ing the overlying rock. 

4. Bedding plane - The surface that sep- 
arates each successive layer in a strati- 
fied body of rock. 

5. Clay dike (clastic dike, clay vein) - 
Many sedimentary formations contain 
transecting tabular bodies of clastic ma- 
terial. These intruding bodies, usually 
called clastic dikes, are composed of ex- 
traneous materials that have invaded the 
containing formation along fissures ei- 
ther from below or above. When the in- 
vading material is composed of clay, the 
dikes are frequently called clay dikes 
or clay veins. In the Western United 
States, the fill material is often called 
"spar;" in the East, "spar" refers to a 
narrow clay vein occurring only near the 
top of the coalbed. 

6. Clay stone - An indurated (hardened) 
clay. 

7. Cleat - A system of joints in a 
coalbed. 

8. Coal split - A coalbed that is sepa- 
rated by rock partings into two or more 
layers that may or may not rejoin some 
distance away. The layer of rock that 
separates the coal. 

9. Concretion - In this report, concre- 
tions are defined as aggregates of min- 
eral material in other sediments such as 



coal balls , Frequently , they have a nu- 
cleus and concentric internal structure. 

10. Crib - A structure composed of 
frames of timber laid horizontally upon 
one another, as in the walls of a log 
cabin (used to support the roof in under- 
ground mines) . 

11. Crossbar (collar, cap, bridge board, 
roof bar) - The horizontal roof member of 
a timber set in mine entries. A horizon- 
tal bar supported by roof bolts. 

12. Cutter - A stress-induced, steeply 
dipping fracture that initiates at the 
rib line and propagates upward into the 
roof rock. 

13. Cutter roof - A coal mine roof that 
is prone to cutter-type failure. 

14. Destressing - The process of reliev- 
ing the pressure or load on rock around 
underground openings . 

15. Draw slate - A weak shale in the 
immediate mine roof that falls when the 
supporting coal is removed, or soon 
thereafter, 

16. Fault - A rock fracture of natu- 
ral origin along which there has been 
displacement. 

17. Full-column resin bolts - Roof bolts 
that are grouted in place in the rock and 
have a column of grout that extends along 
the entire length of the bolt. The terra 
"resin bolt" is a misnomer, and resin re- 
fers to the type of grout; the bolt it- 
self is type 40 steel (or better) . 

18. Head coal (top coal) - Coal that is 
left on the roof of a coal mine for the 
purpose of shielding the roof from the 
effects of exposure to mine air humidity. 

19. Header - A block of wood used under 
the roof bolt plate to increase the 
effective bearing area for installed 
roof bolts. In some minir^g areas, the 
term refers to the block of wood placed 



14 



between the top of a post and the mine 
roof. 



30. Paleochannel - An 
stream channel. 



ancient buried 



20. Horseback - In this report, it re- 
fers to rolls at the top or bottom of a 
coal seam. The term is sometimes applied 
to clastic dikes in coal, large inter- 
secting slickensides in the roof, or fos- 
silized tree trunks. 

21. Induced stress - Rock pressure 
around the mine opening that has been 
caused by excavation of the mine it- 
self or by other mine excavations in the 
vicinity. 

22. In situ stress (far field stress, 
remnant stress) - Rock pressure that was 
present prior to the creation of the mine 
opening. 

23. Joint - A fracture of natural origin 
that is not attended by displacement. 

24. Kettlebottom (pot, bell) - Columnar 
masses of rock in mine roof consisting 
usually of the casts of ancient tree 
stumps. These may drop out of the roof 
without warning. The surface is usually 
highly slickensided and striated. 



31. Pillar punching - When the load on a 
mine pillar exceeds the bearing strength 
of the underlying floor (without causing 
the pillar to fail) , and the pillar is 
pushed into the floor. 

32. Pillar spalling (pillar sloughing) - 
The breaking off of pieces of coal from 
the rib or pillar; the term can include 
minor rib failures. 

33. Pinchout - The wedging out (by lat- 
eral thinning) of one layer of rock be- 
tween two other layers. 

34. Point-load test - A test designated 
to measure crushing strength of a mate- 
rial by using a force applied through two 
opposed, pointed platens (hence point 
load) . Mathematical procedures then are 
used to estimate compressive strengths 
using point-load data. 

35. Posts - Timber placed upright that 
are used to support the mine roof. They 
may be used alone with cap blocks or with 
headers or crossbars. 



25. Laminations - To rock bedding 
layers less than 1 in thick. 



in 



26. Lateral stress - In situ stress that 
is horizontal or near horizontal in 
orientation. 

27. Layer - Any stratum of rock sepa- 
rated from superjacent and subjacent rock 
by a poorly bonded bedding plane. 

28. Mesh (road mesh, wire mesh, weld 
mesh, chain link fence) - Interlaced or 
woven heavy steel wire used to help sta- 
bilize the roof and ribs of mine openings 
or to catch and hold rock that breaks 
away from the roof and ribs. 

29. Multiseam mining - The mining of two 
or more coal seams underlying the same 
surface area. 



36. Pressure arch theory - The pressure 
arch theory states that when an opening 
is driven in a coalbed, the vertical load 
once supported by the extracted mate- 
rial is transferred to the sides of the 
opening. 

37. Override (squeeze, ride over, pres- 
sure override, ride) - Downward and lat- 
eral movement of mine roof accompanied by 
pillar crushing, pillar punching, and 
roof failure, resulting from an exces- 
sive load of overburden. This condition 
usually develops from improper pillar 
extraction. 

38. Parting - A thin sedimentary layer, 
sometimes organic, separating thicker 
rock or coal strata. 



15 



39. Rash - Thinly interlaminated layers 
of shale and coal that sometimes occur 
between the coalbed and the overlying 
rock. 

40. Resin-grouted bolt - A steel bolt 
that is installed by using a resin to 
anchor the bolt in the rock prior to 
tensioning. 

41. Roof truss - An arrangement whereby 
opposite-placed angle bolts are connected 
by a turnbuckle and thereby placed in 
tension, thus exerting an upward compres- 
sive force against the exposed roof. 



45. Seat earth - Stratum underlying a 
coal seam. Commonly a rooted clay-stone. 

46. Slickenside - A polished, striated 
surface caused by differential compaction 
of coal-bearing strata. 

47. Slump - The mass of sediment that 
has slid down from a stream bank into an 
open stream channel. 

48. Snap top - Highly stressed coal mine 
roof, which under some conditions, emits 
audible snapping sounds soon after 
mining. 



42. Roll - A minor protrusion of rock 
into the top or bottom of a coalbed. 
Term can include small flow and compac- 
tion structures and paleochannels . 

43. RQD (Rock Quality Designation) - A 
quantitative index, expressed as percent- 
age, that is based on a recovery proce- 
dure for drill core. It reflects the 
fracturing and softening in a rock mass. 

44. Sealant - Any material that is 
painted or sprayed onto mine roof or 
rib to prevent slaking or spalling. 
Also used on stoppings to seal off air 
leakage. 



49. Stackrock - Thinly interlaminated 
shale and sandstone occurring in coal 
mine roofs. Individual layers in stack- 
rock may lack lateral continuity. 

50. Strap - A corrugated steel sheet (4 
to 15 in wide) against the roof to assist 
in maintaining the stability of the roof. 

51. Tension bleedoff - A decrease in the 
tensile prestress of point anchored, ten- 
sioned bolts that results from creep of 
the anchor. 

52. Underclay - A bed of claystone un- 
derlying a coal seam. See "Seat earth." 



•.-,- U.S. GOVERNMENT PRINTING OFFICE: 1986-605-017/40,028 



INT.-BU.OF MIN ES,PGH.,P A. 28248 



H 20 1 



U.S. Department of the Interior 
Bureau of Mines— Prod, and Distr. 
Cochrans Mill Road 
P.O. Box 18070 
Pittsburgh, Pa. 15236 



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